The host range of and host-cell susceptibility to the avian RNA tumor viruses is dependent on specific interactions between genetically controlled properties of the viral envelope and the host-cell plasma membrane. The cell membrane "receptor" may act as an inherited isoantigen and be detectable by existing serological methods. This proposal is aimed at the serologic identification of erythrocyte isoantigens which may be associated with three different autosomal loci, each controlling susceptibility to a different viral subgroup. Lines of chickens which have been extensively blood typed over a period of several generations for eleven blood group systems are crossed with other lines which have been fully characterized for susceptibilty or resistance to one or more of the three viral subgroups. Chickens matched for known blood types but differing in susceptibility at one of the three viral subgroup loci are used as donors and recipients for immunizations with erythrocytes. Each of the resulting antisera will be tested to determine if the antigen detected is genetically, immunologically or functionally associated with the plasma membrane site for viral penetration. BIBLIOGRAPHIC REFERENCES: Briles, W.E., and E.P. Schelling. (Abstract) Relative influence of marked chromosome segments on chick livability following JM Marek's challenge. Poultry Science, in press, l975. Briles, W.E. and Howard A. Stone. (Abstract) Relative influence of B isoalleles from lines N and P on Marek's mortality. Poultry science, in press, l975.